Spec-Zone .ru
спецификации, руководства, описания, API
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Spec-Zone .ru
спецификации, руководства, описания, API
|
You can move databases or tables from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to different disks.
For InnoDB tables, use the DATA DIRECTORY clause on
the CREATE TABLE statement instead of symbolic links, as explained in Section 5.4.1.2, "Specifying the Location of a
Tablespace". This new feature is a supported, cross-platform technique.
The recommended way to do this is to symlink entire database directories to a different disk. Symlink MyISAM tables only as a last resort.
To determine the location of your data directory, use this statement:
SHOW VARIABLES LIKE 'datadir';
On Unix, the way to symlink a database is first to create a directory on some disk where you have free space and then to create a soft link to it from the MySQL data directory.
shell>mkdir /dr1/databases/testshell>ln -s /dr1/databases/test/path/to/datadir
MySQL does not support linking one directory to multiple databases. Replacing a database directory with a
symbolic link works as long as you do not make a symbolic link between databases. Suppose that you have a
database db1 under the MySQL data directory, and then make a symlink db2 that points to db1:
shell>cdshell>/path/to/datadirln -s db1 db2
The result is that, or any table tbl_a in db1,
there also appears to be a table tbl_a in db2. If
one client updates db1.tbl_a and another client updates db2.tbl_a,
problems are likely to occur.
Symlinks are fully supported only for MyISAM tables. For files used by tables
for other storage engines, you may get strange problems if you try to use symbolic links. For InnoDB tables, use the alternative technique explained in Section
5.4.1.2, "Specifying the Location of a Tablespace" instead.
Do not symlink tables on systems that do not have a fully operational realpath() call. (Linux and Solaris support realpath()). To determine whether your system supports symbolic links, check the
value of the have_symlink
system variable using this statement:
SHOW VARIABLES LIKE 'have_symlink';
The handling of symbolic links for MyISAM tables works as follows:
In the data directory, you always have the table format (.frm) file, the data (.MYD) file, and the
index (.MYI) file. The data file and index file can be moved elsewhere
and replaced in the data directory by symlinks. The format file cannot.
You can symlink the data file and the index file independently to different directories.
To instruct a running MySQL server to perform the symlinking, use the DATA DIRECTORY and INDEX DIRECTORY options
to CREATE TABLE. See Section
13.1.17, "CREATE TABLE Syntax". Alternatively, if mysqld is not running, symlinking can be
accomplished manually using ln -s from the command
line.
The path used with either or both of the DATA
DIRECTORY and INDEX DIRECTORY options may not include
the MySQL data directory. (Bug #32167)
myisamchk does not replace a symlink with the data
file or index file. It works directly on the file to which the symlink points. Any temporary files
are created in the directory where the data file or index file is located. The same is true for the
ALTER TABLE, OPTIMIZE TABLE, and REPAIR TABLE statements.
When you drop a table that is using symlinks, both the
symlink and the file to which the symlink points are dropped. This is an
extremely good reason not to run mysqld as the system root or permit system users to have write access to MySQL
database directories.
If you rename a table with ALTER TABLE ... RENAME or RENAME TABLE and you do not move the table to another database, the
symlinks in the database directory are renamed to the new names and the data file and index file are
renamed accordingly.
If you use ALTER
TABLE ... RENAME or RENAME
TABLE to move a table to another database, the table is moved to the other database
directory. If the table name changed, the symlinks in the new database directory are renamed to the
new names and the data file and index file are renamed accordingly.
If you are not using symlinks, start mysqld with the --skip-symbolic-links option to ensure that no one can use mysqld to drop or rename a file outside of the data
directory.
These table symlink operations are not supported:
ALTER
TABLE ignores the DATA DIRECTORY and INDEX
DIRECTORY table options.
As indicated previously, only the data and index files can be symbolic links.
The .frm file must never be a
symbolic link. Attempting to do this (for example, to make one table name a synonym for another)
produces incorrect results. Suppose that you have a database db1 under
the MySQL data directory, a table tbl1 in this database, and in the
db1 directory you make a symlink tbl2 that
points to tbl1:
shell>cdshell>/path/to/datadir/db1ln -s tbl1.frm tbl2.frmshell>ln -s tbl1.MYD tbl2.MYDshell>ln -s tbl1.MYI tbl2.MYI
Problems result if one thread reads db1.tbl1 and another thread
updates db1.tbl2:
The query cache is "fooled" (it has no way of knowing that tbl1 has not been updated, so it returns outdated results).
ALTER statements on tbl2 fail.
On Windows, symbolic links can be used for database directories. This enables you to put a database directory at a different location (for example, on a different disk) by setting up a symbolic link to it. Use of database symlinks on Windows is similar to their use on Unix, although the procedure for setting up the link differs.
Suppose that you want to place the database directory for a database named mydb
at D:\data\mydb. To do this, create a symbolic link in the MySQL data
directory that points to D:\data\mydb. However, before creating the symbolic
link, make sure that the D:\data\mydb directory exists by creating it if
necessary. If you already have a database directory named mydb in the data
directory, move it to D:\data. Otherwise, the symbolic link will be
ineffective. To avoid problems, make sure that the server is not running when you move the database
directory.
The procedure for creating the database symbolic link depends on your version of Windows.
Windows Vista, Windows Server 2008, or newer have native symbolic link support, so you can create a symlink using the mklink command. This command requires administrative privileges.
Change location into the data directory:
C:\> cd \path\to\datadir
In the data directory, create a symlink named mydb that points to the location of the database directory:
C:\> mklink /d mydb
D:\data\mydb
After this, all tables created in the database mydb are created in D:\data\mydb.
Alternatively, on any version of Windows supported by MySQL, you can create a symbolic link to a MySQL
database by creating a .sym file in the data directory that contains the path
to the destination directory. The file should be named db_name.symdb_name is the database name.
Support for database symbolic links on Windows using .sym files is enabled by
default. If you do not need .sym file symbolic links, you can disable support
for them by starting mysqld with the --skip-symbolic-links option. To determine whether your system supports
.sym file symbolic links, check the value of the have_symlink system variable using this statement:
SHOW VARIABLES LIKE 'have_symlink';
To create a .sym file symlink, use this procedure:
Change location into the data directory:
C:\> cd \path\to\datadir
In the data directory, create a text file named mydb.sym that contains this path name: D:\data\mydb\
The path name to the new database and tables should be absolute. If you specify a
relative path, the location will be relative to the mydb.sym
file.
After this, all tables created in the database mydb are created in D:\data\mydb.
Because support for .sym files is redundant with native symlink
support available using mklink, use of .sym files is deprecated as of MySQL 5.6.9 and support for them will be
removed in a future MySQL release.
The following limitations apply to the use of .sym files for database symbolic
linking on Windows. These limitations do not apply for symlinks created using mklink.
The symbolic link is not used if a directory with the same name as the database exists in the MySQL data directory.
The --innodb_file_per_table option cannot be used.
If you run mysqld as a service, you cannot use a mapped drive
to a remote server as the destination of the symbolic link. As a workaround, you can use the full
path (\\servername\path\).